1. Field of the Invention
This invention relates to the field of liquid separation processes that use membrane systems, and more specifically, to methods and apparatus for determining the state of fouling and/or the state of cleaning of such membrane systems.
2. Description of Related Art
It is known that ultrasonic transducers can be used with ultrasonic pulser/receiver equipment to provide a wide variety of ultrasonic test systems, with emphasis on high-frequency applications, examples being high-resolution flaw detection, thin material thickness gaging, medical research, and materials characterization to measure sound velocity and attenuation, which in turn can be correlated to elastic modulus or grain orientation (size/texture).
The use of membranes in water treatment applications is known, and a joint NWRI/NSF workshop panel has recognized the importance of detecting and monitoring the fouling process with the development of non-destructive, on-line, real-time observation techniques (AWWA Membrane Technology Research Committee, JAWWA, 84(1), 59, (1992)).
The measurement of inorganic fouling using sensor technology based upon acoustic Time Domain Reflectometry (TDR) has been reported in the Review of Progress in Quantitative NDE (Nondestructive Evaluation), 14, 1167, 1995, and other work has indicated the feasibility of utilizing acoustic TDR for measurement of biofouling, Report to the Bureau of Reclamation, Mar. 26, 1996.
Measurement of the fouling and/or cleaning of membranes that are used in liquid separation processes is pivotal to the art if membranes are to be widely used. The lack of suitable techniques for studying membrane fouling under realistic operating conditions has hindered the development of strategies to improve resistance to fouling. Moreover, although a decline in the permeate flow rate (i.e., the treated liquid flow rate) can be used to infer that membrane fouling is occurring, or has occurred, flow rate measurement does not provide a determination of when a membrane module has been adequately cleaned via chemical means, back flushing, or any other cleaning means, since permeation flow does not occur during membrane cleaning, and flow rate measurements are not made during cleaning.
It is known that the fouling of membrane modules can be detected by means of an optical probe. However, use of an optical probe requires that an optical window be formed in a wall of the housing that internally holds the membrane. This procedure is not practical in commercial high-pressure membrane modules. Moreover, optical probes provide information on membrane fouling relative to only on the outermost portion of the membrane. That is, interior membrane portions cannot be seen via an optical probe.
It is also known that an indirect measurement of membrane fouling can be obtained via measurement of the permeate flow rate out of the membrane, an increase in back pressure, or via measurements of the output permeate's composition. Membrane fouling is known to cause a decline in the permeate flow rate. However, permeate flow rates can decline for reasons other than membrane fouling. For example, concentration polarization and membrane compaction also causes the permeate flow rate to decline. Moreover, permeate flow rate measurements cannot be used to assess the effectiveness of cleaning fouled membrane surfaces, since normal permeate flow measurement does not occur during membrane cleaning.
The same criticisms apply to using an analysis of the permeate composition to assess membrane fouling. In particular, permeate composition does not provide appropriate information during the cleaning of fouled membrane modules, since feed composition changes occur during cleaning.
Membrane fouling has been identified as the major factor that limits the implementation of membranes in liquid separation processes. Moreover, membrane fouling limits the time period during which a membrane module can be used before a need to clean the membrane module arises.
It is known that others have used ultrasonic time-domain reflectometry when studying membrane formation and membrane processes. This prior work has involved studies of membrane formation, membrane compaction, membrane fouling, and defect formation in membranes. However, this aforementioned work has not included an apparatus, meter or method for measuring membrane fouling and/or membrane cleaning in membrane modules, such as spiral wound and hollow fiber membrane modules. For example, see an article entitled Real-Time Nondestructive Characterization of Membrane Compaction and Fouling, REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE TESTING, Vol. 14, Plenum Press, 1995, pages 1167-1173, that describes how compaction and fouling were characterized in-situ, and in real-time, using a nondestructive ultrasonic technique relative to a thin film membrane whose structure consisted of a 0.2 micro-meter thick polyamide layer supported by a 40 micro-meter thick polysulfone substrate attached to a 150 micro-meter thick polyester web.
A description of spiral wound membrane modules and hollow fiber membrane modules, as will be discussed in the following detailed description of the present invention, can be found in the publication MEMBRANE HANDBOOK by W. S. Winston and K. K. Sirkar, Van Nostrand Reinhold, 1992.
The present invention satisfies a need in the art for an apparatus/method for detecting the initiation of membrane fouling as well as the state of membrane fouling and the rate of membrane fouling, to thereby provide an early warning that permits adjusting system operating parameters to mitigate the fouling problem.
By providing a measurement of membrane fouling, the present invention satisfies a need in the art for an apparatus/method that determines when a membrane module should be cleaned, as contrasted to techniques that provide measurements that might be a result of factors other than membrane fouling, such as membrane compaction.
This present invention also satisfies a need in the art for an apparatus/method that permits determining when a membrane module has been adequately cleaned via chemical or any other cleaning means. As such, the present invention satisfies a need in the art for an apparatus/method for reducing the amount of cleaning chemicals or other cleaning agents that are required for membrane cleaning, thus also minimizing the down time that is required to clean membrane modules.
The present invention also satisfies a need in the art for an apparatus/method that can be used with membrane modules that are used in a wide range of liquid separation tasks, such as water desalination and reclamation, and the processing of feed streams and waste liquid feed streams.
In general, there is a need in the art for a fouling meter/method that operates to monitor membrane module fouling and/or membrane cleaning, in a non-destructive manner, on-line, in real-time, and non-invasively.